This invention pertains to a method and apparatus for filtering digital data representative of a fluorographic image to obtain edge enhancement or increased sharpness without significant amplification of noise that is inherent in imaging systems.
Digital fluorography procedures are used extensively to permit visualizing the condition of blood vessels in the body. It is basic to digital fluorography to make at least one mask image of the region in the body where the blood vessel of interest resides. This mask image is made and stored before there is any X-ray contrast medium present in the blood vessel. After the contrast medium, which has been injected in the circulatory system, arrives in the region-of-interest another image is made and the mask image is subtracted from it by electronic means. Subtracting yields a difference image wherein everything that remains constant such as the bony structures, between the two images cancels out and anything that has changed, particularly the density of blood vessel due to the opaque medium remains. Thus, the configuration of the blood vessel can be visualized.
When images are simply subtracted there is usually less than optimal sharpness at edges such as where the blood vessel interfaces with surrounding tissue. Hence, it is common practice to resort to edge enhancement in order to provide the sharp or crisp appearing difference image of blood vessels diagnosticians prefer. An undesirable characteristic of prior art enhancement methods is that they enhance the noise that is inherent in the imaging system as well as the edges so the displayed image looks noisy.
It is beneficial to consider the reasons why there is a lack of sharpness in fluorographic X-ray images where transitions are made between adjacent areas that have different densities. In fluorographic systems the X-ray beam or ray bundle that is projected through the body emanates from a point on the X-ray tube target. The X-ray image emergent from the body impinges on the input phosphor of the photocathode of an electronic image intensifier. A ray or plurality of rays that define an edge are in themselves well defined and can be considered points, but they do not produce a corresponding point df light in the input phosphor. Rather, the point or points or incoming ray or rays, respectively, excite slightly larger areas in the input phosphor to fluoresce, thereby contributing to unsharpness. The intensifier converts the input image to a modulated electron beam image which impinges on the output phosphor to yield a bright, visible image. Here again discrete beams of electrons excite zones with intensity gradients on the output phosphor, further contributing to unsharpness.
The optical image on the output phosphor of the image intensifier is viewed with a video camera that converts the image to analog video signals, further spreading of the image points on the target of the camera.
A consequence of the various types of point spreading is that the edges in the fluorographic image are gradual intensity transitions whose representative electric signals do not contain many high frequency components. Thus, in effect, the imaging chain acts like a low pass filtering system.
In fluorographic systems an analog-to-digital converter (ADC) converts the analog video signals to digital signals whose values correspond to the intensities of the pixels that compose the image. The ADC adds broad spectrum noise or white noise. The electronics of the video camera and photonstatistics also add white noise to the signal.
In summary, the high frequency components of the image are attenuated by the image intensifier and video camera while noise, some at high frequencies, is added by the ADC and the video camera. Hence, the digitized image will be noisier than the original X-ray image and the edges of the blood vessel, when displayed on a video monitor will be more diffuse or unsharp or fuzzier. As previously indicated, prior art devices that are designed to enhance or sharpen the edges in images by filtering out the low frequency components in the image signal also amplify the noise undesirably.